Also they are lighthearted and fun and life is short so why not have some fun even when being serious?
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[1] https://knowyourmeme.com/memes/confused-nick-young
[2] https://knowyourmeme.com/memes/scumbag-steveThey aren't fun, don't have anything to do with technical content and just put me off, specially if the presenter thinks s/he could have a 2nd job as stand up comedian.
That's subjective.
>don't have anything to do with technical content
Most public presentations / speeches contain a few jokes. They don't have much to do with the technical content either, but they help people pay attention and ease in the speech.
It's not like we don't have enough boring powerpoint presentations already. It's also not like those going into the trouble of compiling the slides and sharing their knowledge owe us anything...
Most would welcome one or more jokes in a developer speech. So much so, that it's common advice for any kind of public speaking and presentation to add a few jokes to lighten the mood (you can find thousands of articles, books, and public speaking training sessions advising about this).
Some of the best technical speakers add humor in their presentations (often lots of it, e.g. Raymond Hettinger).